A frequency division duplex (FDD) microwave communication system has a transceiver that can simultaneously transmit and receive data for two-way communication. Examples of telecommunication standards that are based on FDD are Universal Mobile Telecommunications System Frequency Division Duplex (UMTS-FDD), Frequency Division Duplex Long Term Evolution (FDD-LTE), Code Division Multiple Access (CDMA), Wideband CDMA (WCDMA), and Asymmetric Digital Subscriber Line (ADSL).
A conventional FDD system generally designated by reference numeral 8 is depicted in FIG. 1. The system 8 has a common antenna 12 and a shared duplexer 14. The transmitter part includes a channel filter 16, digital-to-analog converter 18, low-pass filter 20, transmitter mixer 22, local oscillator 24, and power amplifier 26. The receiver part includes a low-noise amplifier 28, receiver mixer 30, local oscillator 32, low-pass filter 34, analog-to-digital converter 36 and channel filter 38.
On the transmit path, the transmitter part of the transceiver modulates data onto a radio frequency (RF) carrier signal to generate a modulated signal and then amplifies the modulated signal in the power amplifier 26 to obtain a transmit signal of sufficient strength. The transmit signal is routed through the duplexer 14 and transmitted via the antenna 12. On the receive path, the receiver part of the transceiver obtains a received signal via the common antenna 12 and the shared duplexer 14 and then amplifies, filters, and demodulates the received signal to obtain baseband signals. The baseband signals are further processed to recover data from the signal.
In addition to the desired signal, the received signal may include a portion of the transmit signal that leaks from the transmitter via the duplexer to the receiver. Since the transmit signal and the desired signal are typically at two different frequencies, the transmit leakage signal can normally be filtered out by the duplexer 14 and does not pose a problem in itself. However, second order intermodulation (IM2) and higher order intermodulation (IM4, IM6, etc.), collectively known as even order intermodulation, of the leaked transmitter signal may arise due to the nonlinear characteristics of the mixer 30 and low-noise amplifier (LNA) 28 of the receiver. Part of this intermodulation distortion (IMD) may fall within the bandwidth of the desired baseband signal thereby creating noise that may degrade performance. This is particularly important where the communication system seeks to support higher-order modulation, e.g. 8 k-QAM, in which the IM should be, for example, about 40-50 dB lower than the signal.
There is therefore a need in the art for a technique to mitigate, reduce or minimize the deleterious effects of even-order intermodulation distortion in a receiver.